1. Field of the Invention
The present invention relates to an optical disk recording and reproducing device, and particularly to a technology of optimizing a record power when recording data on a recordable optical disk.
2. Description of the Background Art
An OPC (Optical Power Calibration) technology has been proposed. In the OPC technology, test recording of random EFM (Eight to Fourteen Modulation) data is effected on a predetermined area of an optical disk, i.e., an area which is formed in a radially inner portion of the optical disk and is referred to as a PCA “Power Calibration Area”, with a record power at multiple levels, and thereby the record power is optimized.
More specifically, a power function (γ) is calculated from a reproduced RF (Radio Frequency) signal of test record data, and a record power (target record power) exhibiting the γ value of a predetermined target value is calculated. The optimum record power is determined based on this target record power. The γ value is defined by the following equation based on a modulation degree m of the reproduced RF signal and a record power Pw:γ=(dm/dPw)/(m/Pw)  (1)
(dm/dPw) of the right side is a value obtained by differentiating modulation degree m with record power Pw.
In practice, a lead-in area of the optical disk stores a target γ value, a parameter ρ for calculating the optimum record power from the target record power, an erase/record power ratio ε, a coefficient κ for compensating a ratio of (low-speed record erasing power)/(record power) and the like, and these are read from the optical disk to determine the optimum record power and the optimum erase power.
Assuming that a target record power Pwt exhibits the target γ value, an optimum record power Pwo is expressed by the following equation:Optimum record power Pwo=ρ×Pwt  (2)
An optimum erase power (2× and 4× speeds) Peo is expressed by the following equation:Optimum erase power (2× and 4× speeds) Peo=ε×Pwo  (3)
Optimum erase power (1× speed) Peo# is expressed by the following equation:Optimum erase power (1× speed) Peo#=κ×Peo  (4)
These target γ value as well as ρ, ε and κ are determined in advance by an optical disk manufacturer under predetermined conditions, and are recorded on the optical disk.
However, modulation degree m used for calculating the γ value contains an error such as variations in in-plane sensitivity of a record membrane of the optical disk, and this error causes an error in γ value. Therefore, it is difficult to determine uniquely the target record power from the target γ value indicated by the optical disk manufacturer, and this results in a problem that the optimum record power cannot be accurately determined without difficulty.
FIG. 4 represents modulation degree m corresponding to various record powers of a test record data as well as changes in γ value calculated from the modulation degree m according to the equation (1). In FIG. 4, the horizontal axis gives record power Pw during the test recording, the left vertical axis gives the modulation degree m and the right vertical axis gives the γ value. In a prior art, the target γ value is set, e.g., to 1.3. Around this target γ value, as illustrated in FIG. 4, large variations or wavy changes may occur in γ value due to errors in modulation degree m, and it becomes difficult to determine uniquely record power Pw providing the target γ value.
In FIG. 4, P0, P1, P2 and P3 are possible values of the record power that can provide target γ value (γ target), and the optimum record power changes to a large extent depending on the selection of these values. This results in problems that it is difficult to determine the originally optimum record power, and an intended record quality (wavy jitter, error rate and the like) cannot be maintained.
More specifically, when the recording is performed with a record power extremely lower than the originally optimum record power, the jitter and the error rate may be adversely affected. Conversely, when the recording is performed with a record power extremely higher that a primarily or originally optimum value, this adversely affects durability for repetitive recording.
Japanese Patent Laying-Open No. 2003-067925 has disclosed a method of calculating an optimum record power based on an inclination of changes in modulation degree or γ value.
In the method according to the above reference, however, it is necessary to detect the inclination of changes in modulation degree or γ degree, and therefore the record power must be distributed over a wide range to a certain degree so that an excessive load may be exerted on a semiconductor laser.
Since the record power must be distributed over a wide range, a problem that calibration or adjustment takes a long time also arises.